The present invention is related to a system (method and apparatus) for detecting radiation which may be in the form of light that is both visible and invisible to the human eye. Particularly, the invention provides a portable, sensitive detector of light energy. The method and apparatus provided by the invention is capable of detecting with wide dynamic range, invisible light energy such as IR (Infra-Red) as may be found on xe2x80x9clow observablexe2x80x9d aircraft collision avoidance lighting. The system provided by the invention detects light from such sources while discriminating against other light sources which may be present and provide light of much greater intensity which illuminate a single, common detector. Such sources include the sun and taxi ramp lighting.
Generally the invention relates to the detection or measurement of radiation and particularly light energy that may not be normally visible to the human eye in order to easily and quickly ascertain the proper operation of sources (emitters) of such light energy, even if invisible to the unaided human eye. It is a feature of the invention to provide a detection system which is simple, low cost, portable, sensitive and reliable. It is a further feature of the invention to provide a system which detects light from such emitters in the presence of illumination which may be of greater intensity than that provided by the emitter, such as the sun or other lighting and it is capable of distinguishing between the emitter of interest and such other emitters, thus enabling identification of emitters of particular interest.
Historically, the detection or measurement of light has involved the measurement of a voltage or resistance change produced by a material or substance that is sensitive to light energy. Most commonly, semiconductors such a silicon, germanium or other materials such as Cadmium Sulfide. A difficulty arises when attempting to measure a relatively low level of light in the presence of a large level of light. For example, trying to detect a small light emitter while being outdoors in the bright sunlight. Various techniques have been used to increase the sensitivity of the detector in the presence of such bright light. Most common are; shielding or shrouding of the detector, direct placement of the detector over the emitter source, the use of optical filters selected to respond only to the wavelength of the emitter, and the use of multiple sensors that measure the light from various points combined with cancellation circuitry. All of these techniques of detection or measurement are based only upon quantifying the intensity of light received and not the frequency or amplitude modulation characteristics of the light received.
As an example, in verifying the proper operation of IR aircraft collision avoidance lighting, pre-flight inspection of the aircraft can take place on a flight ramp in broad daylight. The IR lighting on the aircraft is many orders of magnitude less intense than the ambient light, thus xe2x80x9cswamping-outxe2x80x9d the very light we are trying to measure. Also, since some of the lighting is not physically accessible, using a detector that requires direct placement over the source is not feasible. In the case of optical filtering, the detector will still allow light within the filter""s bandwidth to reach the detector. In this particular case, the sun""s ambient light has a major portion of its energy within the very bandwidth we wish to measure.
An objective of the invention is to provide a low cost, light detector system that is sensitive to the modulation characteristics of a light source, the invention is especially suitable to detect the emission of IR light from aircraft collision avoidance lighting. Present and future warfare technology capitalizes on night operation. The darkness rendering the forces less visible to ordinary participants. Night vision goggles as used by aircraft pilots are responsive to IR light. Team aircraft are equipped with suitable IR collision avoidance lighting to allow formation flying with other aircraft whose pilots are equipped with night vision goggles. Thus the aircraft lighting is visible to night vision users, but invisible to normal unaided human vision. Preflight inspection of aircraft before flight requires check out of this lighting, often under battle conditions or other such difficult environments. A simple portable hand-held detection device that is easily used by non-technical personnel is needed.
Among the available electrical power on aircraft is 400 Hz AC. If this power source is used to power the lighting, a valuable benefit results in that the light will be modulated with this frequency. Ambient lighting from the sun or taxi ramp is not of this frequency, thus allowing easy discrimination of the light. It is the purpose of this invention to produce a detector system that responds specifically to such modulated light.
Use of this frequency discrimination technique is not limited to aircraft. Various equipment could be equipped with suitable light sources of different frequencies, thus enabling the night vision equipped user to determine what source he is viewing. For example, friendly military tanks could be assigned frequency A, while jeeps frequency B. Thus a night vision equipped pilot or commander could immediately identify such equipment in the darkness of night.